Inhibitors of histone deacetylases (HDAC) have been shown to modulate transcription and to induce cell growth arrest, differentiation and apoptosis. HDAC inhibitors also enhance the cytotoxic effects of therapeutic agents used in cancer treatment, including radiation and chemotherapeutic drugs. Marks, P., Rifkind, R. A., Richon, V. M., Breslow, R., Miller, T., Kelly, W. K. Histone deacetylases and cancer: causes and therapies. Nat Rev Cancer, 1, 194-202, (2001); and Marks, P. A., Richon, V. M., Miller, T., Kelly, W. K. Histone deacetylase inhibitors. Adv Cancer Res, 91, 137-168, (2004). Moreover, recent evidence indicates that transcriptional dysregulation may contribute to the molecular pathogenesis of certain neurodegenerative disorders, such as Huntington's disease, spinal muscular atrophy, amyotropic lateral sclerosis, and ischemia. Langley, B., Gensert, J. M., Beal, M. F., Ratan, R. R. Remodeling chromatin and stress resistance in the central nervous system: histone deacetylase inhibitors as novel and broadly effective neuroprotective agents. Curr Drug Targets CNS Neurol Disord, 4, 41-50, (2005). A recent review has summarized the evidence that aberrant histone acetyltransferase (HAT) and histone deacetylases (HDAC) activity may represent a common underlying mechanism contributing to neurodegeneration. Moreover, using a mouse model of depression, Nestler has recently highlighted the therapeutic potential of histone deacetylation inhibitors (HDAC5) in depression. Tsankova, N. M., Berton, O., Renthal, W., Kumar, A., Neve, R. L., Nestler, E. J. Sustained hippocampal chromatin regulation in a mouse model of depression and antidepressant action. Nat Neurosci, 9, 519-525, (2006).
There are 18 known human histone deacetylases, grouped into four classes based on the structure of their accessory domains. Class I includes HDAC1, HDAC2, HDAC3, and HDAC8 and have homology to yeast RPD3. HDAC4, HDAC5, HDAC7, and HDAC9 belong to class IIa and have homology to yeast. HDAC6 and HDAC10 contain two catalytic sites and are classified as class IIb. Class III (the sirtuins) includes SIRT1, SIRT2, SIRT3, SIRT4, SIRT5, SIRT6, and SIRT7. HDAC 11 is another recently identified member of the HDAC family and has conserved residues in its catalytic center that are shared by both class I and class II deacetylases and is sometimes placed in class IV.
There is still much to be understood about the family of HDACs, including the varying functions of different HDACs and the range of HDAC substrates. In order to learn more about the role that the individual HDACs play, it is important to develop compounds showing selectivity for individual isoforms or small subsets of these isoforms. While some degree of isoform selectivity has been shown by a few compounds, this problem of identifying selective inhibitors is far from solved, and the problem is complicated by the interactions of the HDACs with each other as well as other proteins (cofactors) that can possibly alter their interaction with various inhibitors (Glaser, et al., Biochem. Biophys. Res. Commun., 325, 683-690 (2004). Clinically, the optimal dose, timing and duration of therapy, as well as the most appropriate agents to combine with HDAC inhibitors, are also still to be defined.
The findings to date suggest that HDAC inhibitors have great therapeutic potential in promoting cognitive function, enhancing learning and memory, and treating disease. There is a need to identify specific/selective HDAC inhibitors and to identify the structural features required for potent HDAC inhibitory activity.